Touch screens are typically mounted to cathode ray tube (CRT) displays. They usually include a plurality of LEDs arranged along two orthogonal edges of the CRT display to emit infrared light across the face of the CRT display. A plurality of detectors corresponding to the plurality of LEDs are positioned along the remaining two edges of the CRT display.
To make a selection from the screen, a user contacts a region of the CRT display, thereby breaking at least one of the light beams in each orthogonal direction. The corresponding detectors along the edges detect the broken beams. Each detector corresponds to an x or y-axis coordinate. Thus, by identifying the detectors corresponding to the location selected by the user, the device identifies the x and y coordinates of the region of the CRT display selected by the user.
Such touch screen assemblies typically require each LED in the rows of LEDs to be individually aligned such that the beam of light from each is emitted toward the appropriate detector. The LED is often oriented by hand, such that the central portion of the beam is directed precisely toward the corresponding detector. Touch screen assemblies can therefore be difficult, time consuming and expensive to fabricate.
Such designs are based upon the use of leaded through hole low cost parts; i.e., "T1" LEDs and detectors. The designs take advantage of the flexibility of leaded through hole parts by using the lead spring to orient the devices. Other devices are available, such as RIGHT ANGLE parts which are more expensive. Use of "STANDARD" T1 parts is preferred for cost reasons. Similarly, "T1" style phototransistors are used, because one object of the design is to minimize production costs.
Other approaches to touch screen designs include the use of LEDs that are surface mounted with elaborate lens structures. After the LEDs and detectors are so oriented, their orientation is maintained by their semirigid leads. Over time, the LEDs and detectors may drift from their precise orientation due to vibration, fatigue of the leads, and/or other effects such as temperature. Such failures may require that the assembly be repaired or replaced, increasing costs of operation.
Further, even where the device does not fail due to misorientation of the LEDs and/or detectors, the operation of the touch screen assembly may be impaired by such misalignment. For example, where the central portion of a beam of light from one LED is slightly misdirected, a large portion of the energy from that LED does not reach its corresponding detector. Thus, because the light energy available to the detector is reduced, the sensitivity of the assembly may be impaired. Moreover, some of the misdirected light may impinge upon a detector corresponding to a different LED. The light energy impinging upon such a detector will reduce the sensitivity of the detectory by making it more difficult to detect when the light beam corresponding to that detector is broken.
The cost of repair and maintenance of touch screen assemblies is made more expensive and difficult by their being mounted as part of a CRT assembly. Repair or replacement usually requires the. CRT to be partially disassembled to provide access to the optical elements.